Luminescent films are utilized in the viewing screens of various electronic devices such as cathode-ray tubes, image tubes and electroluminescent (EL) devices. Such films are conventionally made by evaporation, i.e. electron beam evaporation, sputtering or chemical vapor deposition of luminescent materials onto a suitable substrate.
Activated alkaline earth metal sulfides, particularly activated calcium and strontium sulfides, are highly efficient luminescent materials. These materials are conventionally prepared by firing at about 1000.degree. C. which produces a crystalline powder having a particle size of about 5 to 10 micrometers. Such highly efficient crystalline phosphor powders can be utilized in a so-called thick film electroluminescent device.
In the preparation of a thick film electroluminescent device, the crystalline powder phosphor is deposited by settling onto a suitable substrate at room temperature, usually with an adhesion promoting agent, to produce a layer 20 micrometers thick which is loosely adherent and which has considerable voids between the individual particles. The crystallinity of the phosphor is determined by the firing step prior to deposition of the thick film.
In contrast, the phosphor layer in thin film devices is smooth, adherent and approaches the theoretical density of the phosphor material. This infers that the phosphor layer is deposited atom by atom and that it has no memory of its previous form and crystallinity. The crystallinity and stoichiometry, therefore, are totally dependent on the deposition process. In thin film EL devices, the phosphor layers are conventionally formed on the substrate after a plurality of layers of other material, e.g. an electrode and a layer of dielectric material, have been deposited on the substrate. The requisite high temperature firing step precludes the use of inexpensive glass substrates due to their low softening temperatures. The softening point of the useful borosilicate glasses is between about 575.degree. and 600.degree. C., with temperatures up to 650.degree. C. being possible only with special glass formulations.
Heretofore, the only practical low temperature method of forming thin film devices utilizing such crystalline phosphor powders was to sinter the powder at high temperatures to form a crystalline mass which may then be deposited onto the substrate by electron beam evaporation or sputtering. This approach, however, has two serious drawbacks. First, the alkaline earth metal sulfide phosphors are refractory materials and, therefore, are difficult to effectively evaporate. Second, the deposition of these materials by either electron beam evaporation or sputtering has a tendency to cause a loss of sulfur. While this can be compensated for by co-deposition of sulfur, it is extemely difficult to maintain stoichiometry in the deposited film.
In U.S. Pat. Nos. 3,894,164 to J. P. Dismukes et al. and 3,984,587 to S. A. Lipp, there are disclosed chemical vapor deposition (CVD) methods for forming oxide and oxysulfide luminescent films by thermal decomposition of organo-metallic compounds. Such methods are not readily applicable to alkaline earth metal sulfides because very few compounds of calcium, strontium or barium have the necessary volatility for CVD. The same is true for the organic compounds of alkaline earth metals, with the exception of a few chelates.
In copending U.S. patent application, Ser. No. 853,373 entitled "CVD OF LUMINESCENT FILMS AND DEVICES COMPRISING THESE FILMS" filed Apr. 15, 1986 of Kane et al., there is disclosed a method of forming alkaline earth metal sulfide luminescent films by CVD at low temperatures wherein an alkaline earth metal chelate and an activator cation chelate are introduced into a CVD chamber as an aerosol in a nonreactive carrier gas which is admixed with hydrogen sulfide. This method will produce uniform, dense luminescent films as opposed to powders. The crystalline structure of these films, however, is relatively poor. It is therefore desirable to provide a method of producing at low temperature uniform, dense luminescent films of alkaline earth metal sulfide phosphors which have good crystal structure and adherence. Such a method is provided in accordance with this invention.